1. Field of the Invention
This invention relates to a biosignal measuring method and apparatus using a magnetoencephalograph or electroencephalograph. More particularly, the invention relates to a technique for deducing intracerebral activity by removing noise components from detection signals acquired through biosignal measuring sensors.
2. Description of the Related Art
A living body produces minute biomagnetism (biological magnetic fields) as a result of bioelectric currents flowing in the living body. For example, the biomagnetism generated in the brain is called magnetoencephalo which includes induced magnetoencephalo formed by a stimulus applied to the living body, and spontaneous magnetoencephalo produced spontaneously such as alpha wave or spike wave associated with epilepsy.
In recent years, a multichannel SQUID sensor has been developed which uses SQUIDs (Superconducting Quantum Interference Devices) as a fluxmeter for measuring minute biomagnetism produced by the living body. The multichannel SQUID sensor has a multiplicity of SQUID sensors immersed in a coolant such as liquid nitrogen within a vessel called a Dewar.
With a biosignal measuring apparatus or biomagnetism measuring apparatus having the multichannel SQUID sensor (which may be referred to hereinafter as xe2x80x9cfluxmeterxe2x80x9d for short), the fluxmeter is placed adjacent a site of interest, e.g. the head, of a patient. The SQUID sensors in the fluxmeter carry out a noninvasive measurement of minute biomagnetism produced by bioelectric currents occurring in the head, and output magnetism detection signals. Based on the magnetism detection signals from the SQUID sensors, a biomagnetism analysis is performed to determine states of the bioelectric current sources, such as locations, orientations and sizes (see Japanese Patent Publication (Unexamined) H7-327943, for example).
With the conventional biomagnetism measuring apparatus, however, it is difficult to remove noise components sufficiently from the magnetism detection signals provided by the SQUID sensors. The biomagnetism to be measured is extremely weak, and inevitably has, mixed thereinto, noise magnetism (which may be called environmental noise) produced from magnetic sources other than the bioelectric current sources. Thus, the magnetism detection signals from the respective SQUID sensors include noise components due to the noise magnetism having mixed thereinto. An accurate analysis of biomagnetism is not assured without sufficiently removing the noise components from the magnetism detection signals.
It is conceivable to use separate magnetic sensors exclusively for detecting noise magnetism alongside the biomagnetism measuring SQUID sensors. That is, it has been proposed to perform a correction process, using noise magnetism detection signals acquired by simultaneously measuring only noise magnetism, to remove noise components from the magnetism detection signals provided the biomagnetism measuring SQUID sensors.
In this case, the noise magnetism detecting magnetic sensors and the biomagnetism measuring SQUID sensors are installed in different positions, and therefore the noise magnetism detection signals acquired through the noise magnetism detecting sensors are not in precise correspondence with the noise components included in the magnetism detection signals of the biomagnetism measuring SQUID sensors. Thus, the noise components included in the magnetism detection signals of the biomagnetism measuring SQUID sensors have to be deduced from the noise magnetism detection signals acquired through the noise magnetism detecting sensors.
However, it is extremely difficult to determine accurately the noise components in spatially different positions partly because noise magnetism presents a complex aspect. As a result, the noise components cannot be removed sufficiently from the magnetism detection signals of the biomagnetism measuring SQUID sensors.
This invention has been made having regard to the state of the art noted above, and its object is to provide a biosignal measuring method and apparatus for removing noise components sufficiently from detection signals outputted from biosignal measuring sensors.
The above object is fulfilled, according to this invention, by a biosignal measuring apparatus for measuring, with a plurality of sensors, minute biosignals generated from bioelectric current sources in a region of a patient to be diagnosed, the apparatus comprising:
a signal decomposing device for decomposing detection signals provided by the plurality of sensors into a plurality of independent components;
a noise component removing device for determining noise components among the independent components based only on states of the independent components, and removing the noise components;
a signal restoring device for deriving restored detection signals from respective non-noise independent components; and
a signal analyzing device for determining an intracerebral activity (location, orientation and strength) corresponding to each independent component of each of the restored detection signals.
When performing a biosignal measurement with the apparatus according to this invention, the plurality of sensors are first set adjacent a region of a patient to be diagnosed. The sensors pick up minute biomagnetism generated from bioelectric current sources, for example. Detection signals (i.e. original detection signals) provided by these sensors are decomposed into a plurality of independent components by the signal decomposing device. Noise components among the independent components are determined and removed by the noise component removing device. Subsequently, the signal restoring device restores the detection signals (restored detection signals) based only on non-noise independent components remaining after the noise components are removed. The restored detection signals are transmitted to the signal analyzing device. The signal analyzing device performs a biosignal analysis based on the restored detection signals, thereby to determine locations and waveforms of the individual biosignals.
The invention uses the ICA (Independent Component Analysis) technique which decomposes each signal into a plurality of signals with a statistically high degree of independence as noted above. The original detection signals (observation signals) provided by the sensors are decomposed into independent components for respective bioelectric current sources and other sources. Then, it is determined whether or not each independent component is a noise component based only on the state of that component. After removing those independent components determined to be noise components, the detection signals are restored from the remaining, non-noise independent components. Thus, without requiring separate magnetic sensors exclusively for detecting noise, restored magnetism detection signals sufflciently stripped of noise components for the purpose of biosignal analysis are obtained, in the form decomposed into components, only from the signals acquired through the biomagnetism measuring sensors. This allows an accurate signal analysis to be performed based on the restored detection signals.
In the apparatus according to this invention, the noise component removing device, preferably, is operable, with reference to a ratio Ma/Mb between a standard deviation Ma of each independent component for a non-examination period within an entire detection signal measuring time, which non-examination period runs from a start of measurement to a point of time when a stimulus is applied to the patient, and a standard deviation Mb for an examination period following the point of time when the stimulus is applied, to determine an independent component with the ratio Ma/Mb equal to or greater than a fixed value to be a noise component, and an independent component with the ratio Ma/Mb less than the fixed value to be a non-noise component.
Non-noise independent components are generated, for example, after a point of time when a stimulus is applied, and therefore grow after that point of time. Conversely, noise components are not directly related to the period after the point of time when the stimulus is applied, and show little variation across the point of time the stimulus is applied. Consequently, the ratio Ma/Mb between standard deviation Ma for the non-examination period and standard deviation Mb for the examination period is small for the independent components that are true signal components, and large for the noise independent components. Whether an independent component is a noise component or not may be determined by monitoring the value of ratio Ma/Mb.
In the apparatus according to this invention, the signal decomposing device, preferably, is operable to determine and average independent components for each event a plurality of times.
When examining a reaction of the brain to a sound, for example, the averaging of the independent components is effective to remove unwanted components such as quantum noise as well as magnetism resembling spike wave generated from the eye muscles and magnetism due to alpha wave steadily generated from the brain.